The present invention relates to multiple carrier transmission systems.
Multicarrier Modulation, hereinafter denoted as MCM, is also known as Orthogonal Frequency Division Multiplexing, hereinafter denoted as OFDM, or Discrete Multitone Modulation, hereinafter denoted as DMT. It is a technique by which data is transmitted at a high rate by modulating several low bit rate carriers in parallel rather than one single high bit rate carrier. This technique is in principle known from the article of J. J. Nicolas and J. S. Lim, "On The Performance Of Multicarrier Modulation In A Broadcast Multiplath Environment," IEEE ICASSP, Vol. 3, pp 245-248, 1994, hereinafter denoted as "reference 1", and from J. P. Linnartz, S. Hara, "Special Issue On `Multi-Carrier Modulation`," published via internet address http://diva.eecs.berkeley.edu/.about.linnartz/issue.html, hereinafter denoted as "reference 2". It has been shown to be effective for high performance digital radio links and is considered in this report for use with a mobile radio channel.
Fading is often encountered in mobile radio channels where the signal to noise ratio, hereinafter denoted as SNR, across part of the frequency band decreases dramatically for a short period of time. Using a single carrier system a very low error rate can occur between these fades but a very high rate occurs during the fade. This gives an overall error rate which is often unacceptable. MCM deals with these fading characteristics more effectively.
With single carrier modulation an equalizer can be required to reduce the effects of time dispersion. Introducing this means increased noise and gives a transmitter power penalty or increases the systems vulnerability to interference, see reference 2. Since coded MCM has longer symbol intervals than single carrier modulation there is no requirement for an equalizer as known from R. Petrovic, W. Roehr, D. W. Cameron, "Multicarrier Modulation For Narrowband PCS", IEEE Trans. on Veh. Tech., Vol. 43, Iss. 4, pp 856-862, November 1994, hereinafter denoted as "reference 3", and in some cases some time dispersion has been found to actually improve the bit error rate performance of the system, see reference 2. This has been explained by Linnartz due to the reduction of correlation of fading between the carriers because of the diversity present. Only a limited number of subcarriers are subjected to fading at a time and forward error correction coding can deal with the errors on these subcarriers.
Another advantage of MCM is that it is more robust against impulse noise in the time domain as described in the article of T. N. Zogakis, P. S. Chow, J. T. Aslanis and J. M. Cioffi, "Impulse Noise Mitigation Strategies for Multicarrier Modulation", IEEE Int. Conf. on Comms., Vol. 2, pp 784-788, 1993, hereinafter denoted as "reference 4", and has more immunity to fast fades as to be seen from reference 3. An MCM signal can also be tailor made to account for the channels characteristics. For example it can be made to remove certain carriers, thereby avoiding narrowband interference at known frequencies, see reference 1.
OFDM is a form of multicarrier modulation where the subchannel carriers are orthogonal to each other so allowing the use of Fast Fourier Transformation techniques, hereinafter denoted as FFT, and of Inverse Fast Fourier Transformation techniques, hereinafter denoted as IFFT, for the receiver and transmitter functions, eliminating the need for a bank of mixers. A major use of OFDM is in digital audio broadcasting, hereinafter denoted as DAB. Since MCM is robust against multipath fading it will also produce reasonable results if the signals are transmitted from two different transmitter sites where the interference between the two is like that of multipath propagation. This results in efficient use of the radio spectrum which is a major advantage when there is little spectrum available.
MCM can be used for the transmission of low rate video and it has been proposed that it can be used for Digital Video Broadcasting to ensure that mobile signals are reliably received from digital Terrestrial Television broadcasting, see reference 2.
Since OFDM uses a large number of carriers, each of which is modulated by a data signal, and therefore the bit rate associated with each carrier can be made relatively low, the effects of inter-symbol interference due to multipath propagation can be minimised. Conventionally the multi-carrier transmission is generated and demodulated using IFFT and FFT algorithms respectively. This can be computationally expensive, particularly since typical systems can use several megahertz of bandwidth, and the whole of this bandwidth must be sampled and processed.
This proposal is aimed at those applications where it is required to receive (or generate) only a subset of the total number of carriers. This is appropriate where several data signals are multiplexed onto a single broadband transmission. For example, in audio broadcasting it may be desired to receive only one of several audio channels, each of which has been allocated to a number of the available carrier frequencies. In a mobile radio application, a base station may be transmitting data to several users, and the traffic for each user will be partitioned among the available carriers. On the other hand each user will only wish to demodulate their own data signals. In the mobile radio example, the return transmission from each user to the base station may only need a small number of the available carriers.
An advantage of OFDM is that it can provide some degree of frequency diversity in a frequency selective fading environment. This diversity is achieved because although at any given moment some of carriers may be experiencing fading, the others will not. Channel coding can then be applied to correct the transmission errors from the fading carriers, giving Coded OFDM, hereinafter denoted as COFDM. In general, the benefit of frequency diversity is maximised if the carriers used for a particular data channel are spread as far apart as possible in the frequency domain. One convenient way of achieving this is to partition the available carriers into sets of uniformly spaced so called combs, where each data channel is allocated to the carriers of a particular comb. The combs for each channel (or user) are then interleaved in frequency.
Example System
The following example is intended to illustrate the principle. Though in practice other features may also be necessary, it is obvious to a person skilled in the art that this example does not restrict the scope of the present invention to be described later-on in this specification.
For the example a system is considered which transmits 1024 carriers, each of which is modulated e.g. by DPSK at a rate of lkbps. The symbol duration is therefore 1 ms. The carrier spacing can be 1 kHz, which gives a total bandwidth of about 1 MHz. On the premises that e.g. 8 carriers are assigned to a single data channel which for instance has a bit rate of 8 kbps, then a total of 128 channels can be supported. For maximum frequency diversity gain, the carrier spacing for the "comb" of one data channel is then 128 kHz.
It is assumed that in the transmitter there is some means for conversion between a baseband representation of the multicarrier signal and an RF version. It is also assumed that the receiver has means for converting the RF signal to a baseband version, and some means of obtaining frequency and time synchronisation to allow correct demodulation of the data. At the receiver the signal may well be represented by I (in-phase) and Q (quadrature) components. Well known techniques exist for these processes.
In this description effects due to propagation over a non-ideal radio channel will not be considered.
To generate all the carriers would require a 1024 point IFFT to be computed every 1 ms. Similarly to demodulate all the carriers would require a 1024 FFT every 1 ms (as well as other processing). Suitable FFT algorithms can be implemented with available digital signal processing techniques, hereinafter denoted as DSP.
For the given example system the number of arithmetic operations required can be estimated. The basic FFT requires of the order of Nlog.sub.2 N operations, where N is the FFT size. Since these operations are on complex numbers the quantity of operations must be multiplied by some factor (about 4 is reasonable) to convert to real floating point operations as commonly used in measuring the complexity of DSP algorithms. Thus for N=1024, the number of operations in 1 ms is 4*1024*10=40960, which is equivalent to about 41 MOPS (Million Operations Per Second). For comparison, in the literature an efficient implementation of a split radix FFT of 1024 points is quoted as requiring 34774 non-trivial operations, equivalent to about 35 MOPS.